The present invention relates to a process for the recovery of valuable metals from metallic alloys, especially from so-called "furnace residues", which are produced either as by-products in various smelting processes or as the primary product in the recovery of valuable metals from slag (FI Lay-Open Print 56 398) or other materials, during which these valuable metals concentrate in the furnace residue which is forming.
The process according to the invention can be used for treating any metallic materials which contain iron (usually as the main component), cobalt, nickel, molybdenum, copper, gold, silver, platinoids, rhenium, gallium, indium, germanium, tin, arsenic, phosphorus, sulfur, carbon, etc. The process is especially advantageous for the treatment of materials which contain all or several of the above-mentioned elements, but all furnace residue alloys can be treated advantageously by the process according to the invention.
The so-called furnace residues are a well known concept in the pyrometallurgical production of metal, primarily copper. Furnace residue is formed when the reduction potential rises to such a degree that the iron present in the melt is entirely or in part reduced to metal and sinks, owing to its weight, to the bottom of the furnace. Other elements the reduction potential of which is the same as or lower than that of iron are reduced simultaneously with iron. If they dissolve in iron, they tend to concentrate in it, in which case iron serves as a collector. In this manner a high degree of concentration is achieved for those elements which are present in very low concentrations in the primary concentrates, and so the furnace residue may contain considerable amounts of valuable metals. An example of the composition of furnace residue (%) is given in Table 1.
TABLE 1 ______________________________________ Fe Co Ni Cu Mo W Ag Au Re As P S C ______________________________________ 75 1.5 2 10 1.5 0.3 0.03 0.0007 0.01 0.3 1.2 4 0.4 ______________________________________
It can be seen from the above analysis that the concentration of valuable metals is considerably high, and also that iron is the predominant element. When a product according to Table 1 is leached, the solution contains tens of times more iron than cobalt and nickel, and the profitability of the process used for recovering Co and Ni depends on how the Fe is separated. In addition, it can be seen that the composition is very complicated and that a very high selectivity regarding the various elements is required of the process for the recovery of the valuable metals.
The commonly used processes for the separation of iron from solutions include oxidation by means of either air or oxygen, whereby the iron precipitates as an oxy-hydroxide when a neutralizing agent is added. However, such a process produces a large amount of space-consuming precipitate, which results in a considerable washing problem. In addition, the neutralizing agent constitutes a considerable cost even when the least expensive neutralizing agent possible is used.
One method for avoiding the neutralization is to carry out the oxidation in an autoclave at about 200.degree. C., whereby the iron is precipitated as hematite: EQU 2FeSO.sub.4 +1/2O.sub.2 +2H.sub.2 O.fwdarw.Fe.sub.2 O.sub.3 +2H.sub.2 SO.sub.4
According to this reaction about 30-40 g Fe/l can be precipitated as hematite. The Fe.sub.2 O.sub.3 can be used as raw material in iron production, whereby the environmental problem due to the storing of iron precipitate is eliminated. Unfortunately it is, however, difficult to produce Fe.sub.2 O.sub.3 with a sufficiently low sulfur content, since alkaline iron sulfates are also easily precipitated as impurities in iron oxide. The process also requires an expensive and mechanically complicated apparatus.
The iron can also be precipitated as an alkaline sulfate (FeSO.sub.4 OH), whereby at least 100 g Fe/l can be precipitated without neutralization, but in this case, storage of the precipitate, which constitutes an environmental hazard, is necessary.
By the process according to the present invention, it is possible to treat the above-mentioned and similar material in such a manner that the valuable metals can be recovered economically.